Magnetic materials and process of preparation



Sept. 3, 1968 M. w. SHAFER 3,399,957

MAGNETIC MATERIALS AND PROCESS OF PREPARATION Filed Jan. 16, 1968INVENTOR MERRILL W. SHAFER ATTORN EY United States Patent 3,399,957MAGNETIC MATERIALS AND PROCESS OF PREPARATION Merrill W. Shafer,Yorktown Heights, N.Y., assignor to International Business MachinesCorporation, Armonk, N.Y., a corporation of New York Continuation-impartof application Ser. No. 374,321, June 11, 1964. This application Jan.16, 1968, Ser. No. 698,350

4 Claims. (Cl. 2352) ABSTRACT OF THE DISCLOSURE New divalent europiummagnetic materials, having the formula Eu Al O where x=l, 3 or 5, havebeen prepared. A mixture of divalent europium oxide and aluminum oxideis heated in a nonoxidizing atmosphere and cooled. The compounds aretransparent crystalline materials which may be used in magneto-opticaldevices.

This application is a continuation-in-part of my copending application,Ser. No. 374,321 filed on June 11, 1964, now abandoned and entitled,Magnetic Materials and Process of Preparation.

Background of the invention Field of the invention.T his inventionrelates to new europium aluminate magnetic materials and, in particular,to those magnetic materials having the formula Eu Al O where x=l, 3 orand their preparation. In single crystal form, these new magneticmaterials are transparent and thus can be utilized in magneto-opticaldevices.

Description of the prior art Magnetic materials generally fall into twoclasses, those which exhibit cooperative magnetism, and those which donot. Those withthe cooperative magnetic elfect are the ferromagnetic,ferrimagnetic and anti-ferromagnetic materials. The great majority ofthe ferromagnetic materials are metals and good electrical conductors,hence, opaque materials. The ferrimagnetic materials can have a range ofresistivities, but their conduction bands are partially filled and alsoare opaque for all practical purposes. Although the antiferromalgneticmaterials can have unfilled conduction bands, their net magnetic momentsare small and consequently have little practical use.

The existence of insulating ferromagnetic or ferrimagnetic materials arerare since they would require that the conduction band be unfilled andstill exhibit a positive interaction involving unpaired electrons.Examples of these materials wouldhe CrBr Na Fe F and the followingsilicates of europium, Eu SiO Eu SiO On the other hand, the yttrium irongarnets, e.g., Y Fe O and rare earth iron garnets only partially fillthese requirements, since they are only transparent in very thinsections.

It is well known that when a light is passed through a magnetic materialin which there are unpaired electrons, there will be an interaction andthe light will be rotated in a. manner which is dependent on thealignment and the number of unpaired electron spins of the material(i.e., the Faraday effect). This Faraday effect is usually defined interms of the Verdets constant which is the observed rotation in minutesper gauss per centimeter thickness. This is also true in the case ofparamagnetic materials as well as in 'the case of ferromagnetic andferrimagnetic materials. Thus, the greater the number of unpairedelectrons and their ability to align in a magnetic field, the greaterwill :be the specific rotation of the light that the material cantransmit (i.e., the specific rotation 3,399,957 Patented Sept. 3, 1968ice Summary of the invention A group of europium aluminate magneticmaterials has been prepared, the members of which have unpairedelectrons and are transparent in relatively thick layers in the visibleand near visible spectrum (i.e., wavelengths greater than about 4500A.). These materials have the formula Eu Al 0 where x=l, 3 or 5. Thoseeuropium aluminates where x=3 or 5 exhibit a high saturationmagnetization. The europium aluminate 'where x=1 is paramagentic. Allthe europium aluminates are chemically stable under ordinary conditionsand optically transparent in a single crystal form.

It is an object of the invention to prepare europium aluminate magneticmaterials.

It is a further object of the invention to prepare europium aluminatemagnetic materials having the formula Eu Al O J Where x=l, 3 or 5 andwhich are transparent.

Another object of the invention is to prepare a paramagnetic materialhaving the formula EuAl O Still another object of the invention is toprepare a ferromagnetic material having the formula Eu Al O A stillfurther object of the invention is to prepare a ferromagnetic materialhaving the formula Eu Al O Further, another object of the invention is amagnetooptical device utilizing a crystal of Eu Al O where x: 1, 3 and5.

The foregoing and other objects, features and advantages of theinvention will be apparent from the following more particulardescription of a preferred embodiment of the invention, as illustratedin the accompanying drawing.

The drawing is a diagrammatic showing of a magnetooptical deviceutilizing a europium aluminate crystal of the present invention.

In the magneto-optical device shown diagrammatically in the drawing, asingle crystal (or crystal section) 1 of a ferroor paramagnetic europiumaluminate of the present invention is mounted between spaced crossedpolarizing filters (i.e., polarizer 2 and analyzer 3). The crystal isplaced in a magnetic field (e.g., that produced by an electromagnet 4 orby Helmholtz coils). A light source 5 and a photosensitive cell 6 are sodisposed that the light to which the photosensitive cell is exposed isthat which originates at the light source and passes successivelythrough polarizer 2, crystal 1 and analyzer 3. Since the degree ofrotation of the plane of polarized light passing through theparamagnetic crystal is'dependent upon the magnetic field, the amountand orientation of light originating in the light source and passingthrough the polarizer and analyzer and crystal to the photosensitivecell can be varied by varying the strength of the magnetic field of themagnet. In the ferromagnetic region, above magnetic saturation, therotation is independent of the applied magnetic field and the maximumrotation can be obtained.

All these europium aluminate materials have high Verdets constantseven'at room temperature because of their high magnetic susceptibilityand high degree of transparency. As a result of this high Verdetsconstant, all these europium aluminate materials can be used inmagneto-optical devices such as laser beam modulators, light switches,etc. The compounds Eu Al O and Eu A1 O' exhibit a very large Faradayrotation at helium temperature. In addition, the new ferromagneticmaterials, which are insulators, have properties which find applicationin memory elements, transformer cores, or in any device where highresistivity, low coercive force material is desired at low temperatures.

Table I gives the magnetization data showing magnetic susceptibilityversus temperature in degree Kelvin for the three europium aluminates.The saturation magnetization and Curie temperature data are shown in theright colnmn. All measurements were made using standard techniques.

The Eu Al O (where x=1, 3 and 5) magnetic materials are prepared byreacting intimate mixtures of EuO or EuCO or Eu(OI-I) and Al O atelevated temperatures between 800-l600 C. under nonoxidizing conditionssuch as an oxygen free vacuum or any condition in which there is anabsence of oxygen, such as argon, nitrogen, etc., or under reducingatmosphere containing large percentages of hydrogen, e.g., nitrogen withhydrogen added (1-100 percent hydrogen). CO with approximately 20-80percent hydrogen, argon containing 1-100 percent hydrogen and purehydrogen.

The following specific examples represent embodiments of the inventionand, more particularly, disclose the preparation of the europiumaluminate magnetic materials.

Example IEu Al O EuaAlzOu An elemental analysis of the product gave thefollowing results:

Example IIEuAl O 16.8 grams of EuO are intimately mixed with 10.2 gramsof A1 0 This mixture is then placed in a platinum crucible. The crucibleis placed in an oven and the temperature is raised to 1200 C. in a pureargon atmosphere and held at that temperature for 25 hours. The cruciblewas then cooled to room temperature at a rate of 50 C. per minute. Theresultant product EuAl O is a light red powder which is paramagneticdown to 1.8 K. The product is X-rayed. The pattern is shown below.

EuAlzO4 Line 11 III An elemental analysis of the product gave thefollowing results:

Theory Found (percent) (percent) Total Eu 56.4 55. 9=1=0.5 Total Al.20.0 20.1=1=0.2 Eu++ 55.2=1=0.5

Example IIIEu Al O 84 grams of EuO and 10.2 grams of A1 0 are intimatelymixed. This mixture is placed in a carbon crucible r-r r-z rs s s m s ss s w e QIOIOQQOOHWWUI moozoaao An elemental analysis of the productgave the following results:

Theory Found (percent) (percent) Total Eu.-. 80. (l 79. 4;t- 0. 5 TotalAl. 5. 7 6. 1;};0. 2 Eu 79. 6;!z0. 6

The devices of the present invention have been described as made upessentially of a crystal containing Eu Al O which is placed in amagnetic field and associated optical instrumentation in order toutilize the Faraday effect. These devices may be manufactured accordingto the techniques known in the art for the manufacture of analogousdevices embodying other crystal bodies. The best results are obtainedwhen the specific rotation of the light passing through the crystalvaries linearly with the applied magnetic field.

However, the maximum rotation obtainable for this material can beobtained when operated in the ferromagnetic state, i.e., below theirCurie temperature 10 K. for E113Al20 and 6 K. for EllfiAlzOg.

The new europium aluminates of the invention have the formula Eu A1 OWhere x=1, 3 and 5. These aluminates are transparent and thus are usedin magnetooptical devices.

While the invention has been particularly shown and described withreference to a preferred embodiment thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade References Cited UNITED STATES PATENTS 3,282,856 11/1966 Borchardt23-50 X 3,294,701 12/1966 Vogel et al 2350 X OTHER REFERENCES Warsaw etal.: Inorganic Chemistry, vol. 1, August 1962, pp. 719-720.

OSCAR R. VERTIZ, Primary Examiner.

H. T. CARTER, Assistant Examiner.

